1. Field of the Invention
The present invention relates to a WDM (Wavelength Division Multiplexing) transmission apparatus and an optical amplifier control system which are used for a WDM communication network, and more specifically to an optical amplifier control system which supports an optical wavelength ADM (add drop multiplexing) function and an optical path protection function in the WDN communication network.
2. Description of the Related Art
Optical transmission which uses WDM technology has come into practical use in an optical transmission apparatus, and a WDN ring network using an optical ADM apparatus which supports the ADM function, which operates in the unit of optical wavelengths, and the optical path protection switch function is also coming into practical use.
FIG. 1 shows an example of the constitution of a WDM communication system having the ADM function. A signal from an upper stream (fore node) is transmitted using a transmission path 121, and after being amplified by an amplifier (multi-wavelength optical pre-amplifier) 101, said signal is separated into each individual wavelength by DEMUX (wavelength separator) 102; and after channel selection (through/add/drop) is made by an optical switch 103, said signal is multiplexed by MUX (wavelength multiplexer) 104; and said signal is amplified by a post-amplifier (multi-wavelength post-amplifier) 105, and is transmitted to a downstream (next node) by a transmission path 122.
Also, after the signal which is transmitted by the transmission path 124 is amplified by a pre-amplifier (multi-wavelength optical pre-amplifier) 115, said signal is separated into each individual wavelength by DEMUX (wavelength separator) 114; after channel selection (through/add/drop) is made by the optical switch 103, said signal is multiplexed by MUX (wavelength multiplexer) 112; said signal is amplified by a post-amplifier (multi-wavelength optical post-amplifier) 111, and is transmitted to the downstream (next node) by the transmission path 123.
λa which enters the optical switch 103 from a transmission path 125 can be added to the transmission paths 122 and 123 by the optical switch 103, and λb can be drawn out (dropped) from the transmission paths 121 and 124 by the optical switch 103, and can be transmitted to the transmission path 126.
ALC and AGC can be considered as a method of controlling a multi-wavelength amplifier (pre-amplifier or post-amplifier) used for the WDM communication system.
ALC (Automatic Level Control) Mode
Given the condition that multi-wavelength is n, and target output level per wave is Pn, the ALC (Automatic Level Control) mode is the mode of controlling the gain of an optical amplifier so that the total output Po of the optical amplifier may become constant (=n×Pn) regardless of the optical input level of the optical amplifier.
The ALC mode controls the optical amplifier so that the total output level of the optical amplifier may become constant even if the input level of the optical amplifier fluctuates.
Please note that one of the multiplexed wavelengths is cut off in the optical signal which is inputted in the optical amplifier, and the wavelengths other than the one which was cut off are amplified to a greater degree because the ALC mode tries to keep the total output level constant.
Also, when a new wavelength is added to the optical signal which is inputted in the optical amplifier, the optical amplifier output level per wavelength decreases because the ALC mode tries to keep the total output level constant.
In this way, when the number of wavelengths which are inputted in the optical amplifier increases or decreases, the output level of each individual wavelength changes, so that when the output level of each wavelength deviates from the input tolerance of the O/R module which receives this wavelength, the signal makes an error. In prior arts, in order to solve such a problem that the ALC mode had, a method of multiplexing a pilot signal and controlling the pilot signal to keep the level of the pilot signal constant is considered. In this method, however, extra light (pilot signal) must be always kept multiplexed, so it is necessary to mount the parts for transmitting or receiving, multiplexing, and separating the pilot signal, thus causing the cost to be great and making the size of the apparatus large. Also, when the pilot signal is used, there is a problem in that power consumption increases.
Also, there is a method in which an optical supervisory channel (OSC) is provided, the OSC informs each optical amplifier of multiplexed wavelength information, each amplifier changes the target total output level based on this information, and the gain is controlled.
AGC (Automatic Gain Control) Mode
The AGC mode is the mode for keeping the ratio (gain) of the optical input level and the optical output level of the optical amplifier constant.
When the input level of the optical amplifier fluctuates, the output level of the optical amplifier also changes in accordance with the input level because the gain of the optical amplifier is constant. When the wavelength multiplexing number of an optical signal which is inputted in the optical amplifier changes, if the input level of each individual wavelength does not change, the output level of each individual wavelength does not change either because the gain is constant. So, even if a new wavelength is added or deleted, the service using the existing wavelength is not affected.
(1-1) In the WDM system using the ALC mode, there is the following problem in the prior art in which a supervisory control signal of the optical amplifier is transmitted via the OSC to control the gain. When the number of optical wavelengths is changed, each amplifier is informed of that information via the OSC. Each amplifier controls the gain in accordance with the new number of wavelengths based on that information. At that time, however, if the optical signal after the number of the wavelengths has been changed is inputted into the optical amplifier before each amplifier is informed of the new information on the wavelengths which have been added, the optical amplifier output level of each wavelength fluctuates owing to the influence of the ALC mode, and if said level deviates from the receiving light tolerance of the receiving side, that would cause an error.
(1-2) In order to solve the above-mentioned problem, a method of keeping the amplifier output per wavelength constant by multiplexing the pilot signal and controlling the signal level of that pilot signal in a constant level can be considered. In this method, however, extra light (pilot signal) must be always kept multiplexed, so it is necessary to mount parts and circuits for transmitting or receiving the pilot signal, thus causing the cost for the parts and circuits as well as the power to be spent extra.
(1-3) Also, as another method of controlling the optical amplifier, there is a method in which the transmission power of the optical amplifier is monitored, the next downstream node is informed of the monitored transmission power information via the OSC, the transmission power information of the fore node is obtained in the next node, and the gain of the optical amplifier is controlled so that the optical amplifier output of said node may become the same as the transmission power of the fore node. However, in the event that the number of wavelengths increases or decreases in the fore node and the transmission power changes, if an optical signal whose number of wavelengths has been changed is inputted into the optical amplifier before the next node is informed of the new transmission power information, the gain of the optical amplifier cannot be correctly controlled, and if said output level deviates from the receiving light tolerance on the receiving side, that would result in an error or the incapability of receiving the information, thus affecting the service for which said wavelength is used.
(2) In the WDM system of prior art in which the gain of the optical amplifier is controlled in the AGC mode, there is the following problem. In the operation of the WDM system based on the AGC mode, when the WDM system is started, it is necessary to measure the optical loss between the nodes and set the amplifier gain so as to compensate for the loss. However, in the measurement which is carried out using measuring equipment, the loss changes between the state where the measurement equipment is connected and the state where connectors are engaged in a practical use, so it is not easy to accurately measure the loss including the loss in the connector junction. If the accuracy of measuring the optical loss is bad, it is necessary to consider an extra level margin when originally designing an optical line. Therefore, there is plenty of demand that the booting (gain setting) of the optical amplifier including the optical loss measurement should be automated.
(3) Also, there is another problem as follows. The optical loss of the optical fiber used for connection between the nodes changes due to environmental conditions such as temperature and physical bending, so in the WDM system which controls the gain of the optical amplifier in the AGC mode, an extra level margin must be considered for such an optical loss when the optical line is designed, the design of the optical line becomes all the more difficult, and the optical amplifier whose output for absorbing these levels of changes is large and a VAT (variable optical attenuator) whose attenuation is large must be used, inevitably making the cost of the parts high.
(4) FIG. 2 shows an example of the WDM communication network. In the event that any fault occurs to the clockwise working path in the WDM ring network shown in FIG. 2, when a path protection switch for switching an optical wavelength to a counter-clockwise protection path is realized, the path of the optical wavelength is switched over by the protection switch, so that the number of wavelengths which are inputted into the optical amplifier changes from 0 to the number of wavelengths to be protected, and the optical input level changes rapidly. In the path through which an optical signal usually does not pass, like a protection path, since there is no optical input or output level, the optical amplifier cannot be controlled by ALC or AGC, so the optical amplifier must be kept shutdown. However, there is a problem in that it takes time to start the optical amplifier from a shutdown state when the path of the optical wavelength is switched over by the protection switch.